1. Field of the Invention
The present invention relates to a catalyst for purifying exhaust gases which is suitable for eliminating nitrogen oxides (NO.sub.x) from exhaust gases from automobiles, boilers and the like.
2. Description of the Prior Art
For elimination of nitrogen oxides from exhaust gas, a number of methods have been known heretofore. The known methods include (1) a selective reduction method using ammonia in the presence of a V.sub.2 O.sub.5 --TiO.sub.2 catalyst, (2) an automotive three-way catalyst method in which a Pt-Rh/Al.sub.2 O.sub.3 catalyst is used, and (3) a direct decomposition method which uses a noble metal catalyst or metal oxide catalyst, such as Pt/Al.sub.2 O.sub.3, Co.sub.3 O.sub.4, YBa.sub.2 Cu.sub.3 O.sub.y and the like. These known methods, however, have respective drawbacks and are therefore unsatisfactory. That is, the method (1) involves difficulties as to cost and equipment because of the use of ammonia, whereas the catalysts used in the methods (2) and (3) can hardly catalyze the desired reactions for elimination of nitrogen oxides where the exhaust gas to be treated contains oxygen in excess of the stoichiometric quantity for the reaction system.
Recently, therefore, many research efforts have been made on copper ion-exchange zeolite catalysts capable of exhibiting relatively high activity even in the presence of excess oxygen. For example, there have been reports on a copper-transition metal co-ion-exchange zeolite catalyst (Japanese Pre-examination Patent Publication (KOKAI) No. 1-130735), a copper-noble metal co-ion-exchange zeolite catalyst (Japanese Pre-examination Patent Publication (KOKAI) No. 1-310742), a copper-rare earth co-ion-exchange zeolite catalyst (Japanese Pre-examination Patent Publication (KOKAI) No. 3-89942), and the like. These catalysts have a common feature that active metal components are supported on a carrier by an ion-exchange method, and it is reported that supporting the active metal components on the carrier by other method than ion-exchange results in a lowered activity.
Catalysts for purifying exhaust gas which are used to eliminate nitrogen oxides from exhaust gas are required to have high activity and high-temperature durability in the presence of both oxygen and water vapor. However, the copper co-ion-exchange zeolite catalysts, copper-noble metal co-ion-exchange zeolite catalysts, and copper-rare earth co-ion-exchange zeolite catalysts as described above have the drawback that, because the active metal components such as copper are supported on the carrier by an ion-exchange method, the principal active component element is supported only in a limited amount and, besides, stable supporting of copper on zeolite is not achieved. Accordingly, when these catalysts are exposed to high temperatures, aggregation of copper will occur, making it difficult to maintain sufficient activity. In particular, the presence of water vapor tends to cause further lowering in the activity of the catalysts. Due to the poor high-temperature durability in the presence of oxygen and water vapor, the copper ion-exchange zeolite catalysts have not yet been put to practical use.